The
electron
was first discovered in 1898 by Sir John Joseph Thomson. Almost 100 years
ago J.J. Thomson was at Cavendish Laboratory at Cambridge University. Thomson
was investigating 'Cathode Rays' which had been a puzzle for a long time.
Through his experiments Thomson put forward a then controversial theory
in which the 'Cathode Rays' were made up of streams of particles much smaller
than atoms, Thomson called these particles 'corpuscles'. Thomson mistakenly
believed that these 'corpuscles' made up the entire atom. This idea was
controversial as most people at this time thought that the atom was the
smallest particle in matter and was divisible.

Thomson's
theory was not explicitly supported by his experiments. It took more experimental
work by Thomson and others to conclusively prove the theory. The atom is
now known to contain other particles as well. Yet Thomson's bold suggestion
that 'Cathode Rays' were material constituents of atoms turned out to be
correct. The rays are made up of electrons: very small, negatively charged
particles.

Mysterious
Rays

Science
lecturers who traveled from town to town in the mid nineteenth century
delighted audiences by showing them the ancestor of the neon sign. They
took a glass tube with wires embedded in opposite ends. They then applied
a high voltage across the tube. When the air was pumped out of the tube
the interior of the tube would glow. In 1859 a German physicist sucked
out still more air with an improved pump and saw that where this light
from the cathode reached the glass it produced a fluorescent glow. Evidently
some kind of ray was emitted by the cathode and lighting up the glass.

One theory
was that the rays were waves traveling in an invisible fluid called the
"ether." At that time, many physicists thought that this ether was needed
to carry light waves through apparently empty space. Another possibility
was that cathode rays were some kind of material particle. Yet many physicists,
including J.J. Thomson, thought that all material particles themselves
might be some kind of structures built out of ether, so these views were
not so far apart

Experiments
were needed to resolve the uncertainties. When physicists moved a magnet
near the glass, they found they could push the rays about. But when the
German physicist Heinrich Hertz passed the rays through an electric field
created by metal plates inside a cathode ray tube, the rays were not deflected
in the way that would be expected of electrically charged particles. Hertz
and his student Philipp Lenard also placed a thin metal foil in the path
of the rays and saw that the glass still glowed, as though the rays slipped
through the foil. Didn't that prove that cathode rays were some kind of
waves?

In 1897,
drawing on work by his colleagues, J.J. Thomson set out to prove his theory
by performing three experiments.

The
1897 Experiments

The
First Experiment

In a variation of an 1895 experiment by Jean Perrin, Thomson built a cathode
ray tube ending in a pair of metal cylinders with a slit in them. These
cylinders were in turn connected to an electrometer, a device for catching
and measuring electrical charge. Perrin had found that cathode rays deposited
an electric charge. Thomson wanted to see if, by bending the rays with
a magnet, he could separate the charge from the rays. He found that when
the rays entered the slit in the cylinders, the electrometer measured a
large amount of negative charge. The electrometer did not register much
electric charge if the rays were bent so they would not enter the slit.
As Thomson saw it, the negative charge and the cathode rays must somehow
be joined together.

The
Second Experiment

All
attempts had failed when physicists tried to bend cathode rays with an
electric field. Now Thomson thought of a new approach. A charged particle
will normally curve as it moves through an electric field, but not if it
is surrounded by a conductor (a sheath of copper, for example). Thomson
suspected that traces of gas remaining in the tube were being turned into
an electrical conductor by the cathode rays themselves. To test this idea,
he took great pains to extract nearly all of the gas from a tube, and found
that now the cathode rays did bend in an electric field after all.

J.J. Thomson concluded from these two experiments, "I can see no escape
from the conclusion that cathode ray] are charges of negative electricity
carried by particles of matter." But, he continued, "What are these particles?
Are they atoms, or molecules, matter in a still finer state of subdivision?"

The
Third Experiment

Thomson's third experiment sought to determine the basic properties of
the particles. Although he couldn't measure directly the mass or electric
charge of such a particle, he could measure how much the rays were bent
by a magnetic field, and how much energy they carried. From this data he
could calculate the ratio of the electric charge of a particle to its mass
(e/m). He collected data using a variety of tubes and using different gases.

Thomson's
Results

The results
of Thomson's were astounding. Just as Emil Wiechert had reported earlier
that year, the mass-to-charge ratio for cathode rays turned out to be over
one thousand times smaller than that of a charged hydrogen atom. Either
the cathode rays carried an enormous charge (as compared with a charged
atom), or else they were amazingly light relative to their charge.

Philipp
Lenard settled the choice between these possibilities. Experimenting on
how cathode rays penetrate gases, he showed that if cathode rays were particles
they had to have a very small mass-far smaller than the mass of any atom.
The proof was far from conclusive. But experiments by others in the next
two years yielded an independent measurement of the value of the charge
(e) and confirmed this remarkable conclusion

Thomson
announced the hypothesis that "we have in the cathode rays matter in a
new state, a state in which the subdivision of matter is very much further
than in the ordinary gaseous state: a state in which all matter... is of
one and the same kind; this matter being the substance from which all the
chemical elements are built up."

From
his three experiments in 1897 Thomson presented three hypotheses about
cathode rays

Cathode rays are charged particles
(which he called 'corpuscles').

These corpuscles are constituents
of the atom.

These corpuscles are the only
constituents of the atom.

Thomson's
hypotheses met with some skepticism. The second and third hypotheses were
especially controversial. Years later he recalled "At first there were
very few who believed in the existence of these bodies smaller than atoms.
I was even told long afterwards by a distinguished physicist who had been
present at my lecture at the Royal Institution that he thought I had been
'pulling their legs.'

The word
'electron' first used by G. Johnstone Stoney in 1891 had been used to denote
the unit of charge found in experiments that passed electric current through
chemicals. In this sense Joseph Larmor, J.J. Thomson's Cambridge classmate,
used the term. Larmor devised a theory of the electron that described it
as a structure in the ether. But Larmor's theory did not describe the electron
as a part of the atom. When the Irish physicist George Francis FitzGerald
suggested in 1897 that Thomson's corpuscles were really 'free electrons'
he was actually disagreeing with Thomson's hypotheses. FitzGerald had in
mind the kind of 'electron' described by Larmor's theory.Gradually scientists accepted
Thomson's first and second hypotheses, although with some subtle changes
in their meaning. Experiments by Thomson, Lenard, and others through the
crucial year of 1897 were not enough to settle the uncertainties. Real
understanding required many more experiments over later years.

Theories about the atom proliferated following Thomson's 1897 work. Thomson
proposed a model, sometimes called the 'plum pudding' model, in which thousands
of tiny, negatively charged 'corpuscles' swarm inside a sort of cloud of
mass less positive charge. This theory was disproved by Thomson's former
student, Ernest Rutherford. Using a different kind of particle beam, Rutherford
found evidence that the atom has a small core, a nucleus. Rutherford suggested
that the atom might resemble a tiny solar system, with a massive, positively
charged centre circled by only a few electrons. Later this nucleus was
found to be made up of new kinds of particles (protons and neutrons), much
heavier than electrons.

Milikan's
Experiment

In 1911 Robert Milikan set
out to try and determine the charge of an electron. He did this by balancing
charged oil droplets in an electric field, using the equipment shown below.

Milikan
used a microscope to observe the oil droplets between the plates. When
there is no P.D. between the plates the droplets fall at a steady speed.
However when a P.D. is applied between the plates the oil droplets do one
of three things

Droplets with an overall negative
charge fall more slowly or even stop moving.

Droplets with an overall positive
charge fall more quickly

Droplets with no charge are
not affected.

These droplets
are charged as they are forced out of the nozzle. As the plate voltage
increases some of the drops fall more and more slowly until the drops stop
moving, at this point the electric fore is equal to the weight of the oil
droplet. The electric force on the droplet is given by the charge (q) multiplied
by the electric field strength (V/d).

q=Droplet
Charge V= Holding Voltage d= Distance between plates

m=droplet mass g=
Acceleration due to gravity.

From
his experiments Milikan determined that the charge on an electron was 1.6×10-19
C. In order
for Milikan to determine the mass of an oil droplet accurately he found
that when the P.D. across the plate was off, the speed that oil droplets
fell at was determined only by the mass of the oil droplets. So by timing
how long it takes for a droplet to fall with the plates off he could calculate
the mass of the droplet.

Milikan
noticed that the droplets fell at a constant speed, which meant that the
weight of the droplet was balanced by the viscous force of the droplet
falling through the air. The viscous force on an object is given by the
formula.

F=
Viscous force h=
Viscosity of fluid R= Sphere radius

v= Sphere speed

This gives the formula

However the mass of an object
is also given by the density of the object multiplied by the volume of
the object. Therefore

Determining
e/m

What
follows is our attempt to measure e/m at Egglescliffe School:

Another way of determining e/m is to use the fine beam tube method. In
this method electrons are produced by thermionic emission, and then accelerated
inside a sphere containing nitrogen gas at a low pressure. When the electrons
strike a nitrogen molecule they cause it to emit green light. Either side
of the sphere there are a pair of magnetic coils, placed to provide a uniform
magnetic filed inside the sphere. The magnetic field causes the electrons
to be deflected, if the field is strong enough then the electrons will
orbit in a circle. The gas is at low pressure so as not to scatter the
electrons so they will not form a beam

The electrons
inside the tube are accelerated by a potential difference V (supplied by
the h.t. unit) between the cathode and the conical anode. The kinetic energy
gained by the electrons when they are accelerated is given by the formula

The electrons
are in circular motion so by Newton's Second Law applied radially

The only
source of the force on the electrons is the magnetic force caused by the
interaction of the electrons with the magnetic field, this force can be
given by the formula

By combining these two
we come up with the formula

This can be rearranged
to give

Then substitute this into the
formula giving the kinetic energy of the electrons. After rearranging :

The strength of the magnetic
field (B) can be calculated by the formula

Electron
Microscope

The magnifying power of an optical microscope is limited by the wavelength
of visible light. An electron microscope uses electrons to "illuminate"
an object; since electrons have a much smaller wavelength than light, they
can resolve much smaller structures than light can. The smallest wavelength
of visible light is about 4,000 angstroms (1 angstrom is 1×10-10
meters) the wavelength of electrons used in electron microscopes is usually
about .5 angstrom.

All electron
microscopes comprise several basic elements. They have an electron gun
emitting electrons that strike the specimen and create a magnified image.
Magnetic 'lenses' that create magnetic fields are used to direct and focus
the electrons, because the conventional lenses used in optical microscopes
to focus visible light do not work with electrons. A vacuum system is an
important part of any electron microscope. Electrons are easily scattered
by air molecules, so the interior of an electron microscope must be at
a very high vacuum. Finally, electron microscopes also have a system that
records or displays the image produced by the electrons.

There
are two basic types of electron microscopes: the transmission electron
microscope (TEM), and the scanning electron microscope (SEM). In a TEM
the electron beam is directed on to the object to be magnified. Some of
the electrons are absorbed or bounce off the specimen others pass through
and form a magnified image of the specimen.

The sample
must be cut very thin to be used in a TEM usually the sample is no more
than a few thousand angstroms thick. A photographic plate or fluorescent
screen is placed beyond the sample to record the magnified image. Transmission
electron microscopes are capable of magnifying an object up to 1 million
times.

A scanning electron microscope creates a magnified image of the surface
of an object. When using an SEM, the object to be magnified does not need
to be thinly sliced; the sample can be placed in the microscope with little,
if any, preparation. An SEM scans the surface of the sample bit by bit,
in contrast to the TEM, which looks at a relatively large part of the object
all at once. In an SEM, a tightly focused electron beam moves over the
entire sample, much the way an electron beam scans an image on to the screen
of a television.

Electrons
in the tightly focused beam might scatter directly off the sample, or cause
secondary electrons to be emitted from the surface of the sample, these
scattered or secondary electrons are collected and counted by an electronic
device located to the side of the sample. Each scanned point on the sample
corresponds to a pixel on a television monitor; the more electrons the
counting device detects, the brighter the pixel on the monitor is. As the
electron beam scans over the entire sample, a complete image of the sample
is displayed on the monitor. Scanning electron microscopes can magnify
objects 100,000 times or more. SEMs are particularly useful because, unlike
TEMs and powerful optical microscopes, SEMs produce detailed pictures of
the surface of objects, providing a realistic three-dimensional image.

Various
other electron microscopes have been developed. A scanning transmission
electron microscope (STEM) combines elements of an SEM and a TEM, and can
resolve single atoms in a sample. An electron probe microanalyser, which
is an electron microscope fitted with an X-ray spectrum analyzer, can examine
the high-energy X-rays that are emitted by the sample when it is bombarded
with electrons. Because the identity of different atoms or molecules can
be determined by examining their X-ray emissions, electron probe analyzers
not only provide a magnified image of the sample as a conventional electron
microscope does, but also information about the sample's chemical composition.

Appendix
A - Results from e/m experiment

Va
(Volts)

r
(m)

I
(A)

B
(T) ×10-4

e/m
(CKg-1) ×1011

133

.02

.38

15.72

2.69

110

0.0125

0.65

26.88

1.80

161

0.0425

0.18

7.45

3.14

329

0.0100

1.25

51.70

2.46

118

0.0100

0.68

28.13

2.98

232

0.0375

0.24

9.93

3.26

327

0.0375

0.28

11.58

3.38

The accepted value of e/m
is 1.76×1011 CKg-1.It is hard to get accurate
results from this method as it is difficult to accurately measure the radius
of the circle of electrons, and also the fine beam tube that I was using
had lost some of it's hydrogen making it hard to see the cathode rays.

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contributed by Peter Richards (Yr 13 student 1999-2000)